May 15, 2024
A young box tree moth larva.
Box tree moth: A growing concern for Ontario gardens
Box tree moth (Cydalima perspectalis) has become a significant pest in Ontario’s urban landscapes. The larvae are known for their voracious feeding habits, consuming the foliage of boxwood plants which are widely cultivated in southern Ontario for their aesthetic appeal and structural use in gardens and landscapes. The good news is that there are measures gardeners can take to protect their plants.
Landscape Ontario has been following this pest closely since it was initially discovered in Toronto August 2018 — long before Ontario was declared an infested area in 2023. We have worked in partnership with the Canadian Food Inspection Agency (CFIA) and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) to scout and identify box tree moth populations and develop best management practices to reduce impact and slow the spread of this invasive species.
This time of year, box tree moth larvae are actively feeding — and it’s their voracious appetite that can cause damage or destruction to boxwood shrubs. The most effective way to minimize feeding damage and protect boxwood plants is by spraying a biological product with the active ingredient Bacillus thuringiensis (commercially called Dipel or Safer’s BTK) during periods when larvae are feeding. The active ingredient of these products is a naturally occurring bacteria that is safe to humans and pets, and they are readily available in garden centres across Ontario. In Southern Ontario, there are typically two periods when the box tree moth is in the caterpillar or larval stage and can be managed:
Key recommendations
As the box tree moth continues to spread, collaboration among landscape horticulture professionals, homeowners, and local authorities is crucial. By staying informed and implementing effective management strategies, we can mitigate the impact of this invasive pest on Ontario's urban landscapes.
Landscape Ontario has been following this pest closely since it was initially discovered in Toronto August 2018 — long before Ontario was declared an infested area in 2023. We have worked in partnership with the Canadian Food Inspection Agency (CFIA) and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) to scout and identify box tree moth populations and develop best management practices to reduce impact and slow the spread of this invasive species.
This time of year, box tree moth larvae are actively feeding — and it’s their voracious appetite that can cause damage or destruction to boxwood shrubs. The most effective way to minimize feeding damage and protect boxwood plants is by spraying a biological product with the active ingredient Bacillus thuringiensis (commercially called Dipel or Safer’s BTK) during periods when larvae are feeding. The active ingredient of these products is a naturally occurring bacteria that is safe to humans and pets, and they are readily available in garden centres across Ontario. In Southern Ontario, there are typically two periods when the box tree moth is in the caterpillar or larval stage and can be managed:
- Early to mid-May (activity has usually coincided with when lilacs are in bloom and days are consistently warm).
- Early July to mid-August.
- September before they prepare to hibernate for the winter.
Signs you have box tree moth
- Skeletonization: Box tree moth caterpillars feed voraciously on boxwood leaves, leaving behind skeletonized foliage.
- Defoliation: Severe infestations lead to defoliation, weakening the plants and making them susceptible to other stressors.
- Silken webbing: Similar in appearance to spider nests, box tree moth larvae spin a fine web around a couple of boxwood leaves in the fall, creating a small protected ‘house’ for the larva to overwinter. These can be difficult to find but are usually found deeper within the foliage.
- Life stages present: If you see larvae (green caterpillars with a black head) or adult moths (white wings with light brown margins), it is clear you have box tree moth. Adult moths are nocturnal and are not often found within the boxwood foliage, so the life stages you’re more likely to observe are larvae, and in some cases, pupae.
What you can do
During key points of these larval periods, like now, we urge homeowners, professional landscapers and gardeners to be vigilant in protecting their boxwood shrubs from the invasive box tree moth.Key recommendations
- Inspection: From May through September, regularly inspect your boxwoods for signs of box tree moth infestation. Look for green caterpillars with black heads as well as chewed leaves, webbing and frass pellets (larval excrement). If you spot any signs of the insect, take immediate action.
- Manual removal: If you find any life stages while inspecting your boxwood plants, you can remove them by hand and destroy them. Place them in soapy water, alcohol, or squish them.
- Biological control: When dealing with more than a few larvae, you can combat larval feeding on boxwood plants with a biological pesticide containing Bacillus thuringiensis subsp. Kurstaki, commonly known as BTK (found at your local garden centre). The best times to treat the larvae are when they are less than one-inch long, typically in mid-May, early July, and August-September. By inspecting the plants and observing the small green caterpillars in the foliage, you will know that is the right time to spray. Boxwood plants rarely host other caterpillars, so it is safe to spray your boxwood plants with BTK without impacting other beneficial insects or pollinators.
- Professional assistance: Consider reaching out to a local professional landscape company for expert guidance on treatment options.
- Reporting: If you suspect a box tree moth infestation outside of Ontario, report it promptly to CFIA.
Impact
- Aesthetic damage: Skeletonized leaves compromise the aesthetic appeal of boxwood hedges and topiaries.
- Economic loss: Landscapers, nurseries, and homeowners face increased costs for pest management and replacement of damaged plants.
- Environmental concerns: The loss of boxwood plants in the urban landscape decreases valuable habitat supporting biodiversity.
As the box tree moth continues to spread, collaboration among landscape horticulture professionals, homeowners, and local authorities is crucial. By staying informed and implementing effective management strategies, we can mitigate the impact of this invasive pest on Ontario's urban landscapes.
Box tree moth resources
- landscapeontario.com/box-tree-moth
- Box tree moth help - Landscape Ontario
- Box tree moth: Look for these signs this spring - Landscape Ontario
- Safe removal of boxwood affected by box tree moth - Landscape Ontario
- Box tree moth life cycle infographic - Landscape Ontario
- Box tree moth - Best Management Practices
- Box Tree Moth: 2023 Training & Resources - ONnurserycrops Blog
- iNaturalist observations